The quadratus lumborum is a muscle of the posterior abdominal wall. It is the deepest abdominal muscle and commonly referred to as a back muscle. It is irregular and quadrilateral in shape and broader below than above.

Occasionally, a second portion of this muscle is found in front of the preceding. It arises from the upper borders of the transverse processes of the lower three or four lumbar vertebræ, and is inserted into the lower margin of the last rib.

The quadratus lumborum, or QL, is a common source of lower back pain.[1] Because the QL connects the pelvis to the spine and is therefore capable of extending the lower back when contracting bilaterally, the two QLs pick up the slack, as it were, when the lower fibers of the erector spinae are weak or inhibited (as they often are in the case of habitual seated computer use and/or the use of a lower back support in a chair). Given their comparable mechanical disadvantage, constant contraction while seated can overuse the QLs, resulting in muscle fatigue.[2] A constantly contracted QL, like any other muscle, will experience decreased blood flow, and, in time, adhesions in the muscle and fascia may develop, the end point of which is muscle spasm.

This chain of events can be and often is accelerated by kyphosis, which is invariably accompanied by rounded shoulders, both of which place greater stress on the QLs by shifting body weight forward, forcing the erector spinae, QLs, multifidi, and especially the levator scapulae to work harder in both seated and standing positions to maintain an erect torso and neck. The experience of "productive pain" or pleasure by a patient upon palpation of the QL is indicative of such a condition.

Hip abduction is performed primarily by the hip abductors (gluteus medius and minimus). When the gluteus medius/minimus are weak or inhibited, the TFL [Tensor fasciae latae] or QL will compensate by becoming the prime mover. The most impaired movement pattern of hip abduction is when the QL initiates the movement, which results in hip hiking during swing phase of gait. Hip hiking places excessive side-bending compressive stresses on the lumbar segments. Thus, a tight QL may be another hidden cause of low back pain (Janda 1987).

When the hip adductors are tight or hypertonic, their antagonist (gluteus medius) may experience reciprocal inhibition. The gluteus medius will become weak and inhibited. This in turn may cause hypertonicity of ipsilateral QL. Chronic hypertonicity of QL tends to cause low back pain due to its ability to create compressive stress on lumbar segment. Current studies show that application of heat or ice, massage, and estim will not leave long-term benefits. Careful assessment of muscular imbalances and movement impairments by a therapist is recommended in order to address the underlying issues mentioned.[3]

While stretching and strengthening the QL are indicated for unilateral lower back pain, heat or ice applications as well as massage should be considered as part of any comprehensive rehabilitation regimen.[4]